Often it is desirable to have a space vehicle fly-by another space vehicle. A fly-by vehicle is required to approach and fly-by an object vehicle or planetary object at a predetermined close distance, without intercepting the vehicle or object. One example of this application is space object surveillance. To obtain a good resolution photograph of a satellite requires that the observation camera be close to the satellite. However, the fuel required to rendezvous with a satellite is costly due to the high orbital velocities involved. It is much easier to fly-by than rendezvous with a satellite. A fly-by guidance algorithm is necessary to insure that the proper close approach distance is achieved and that the space vehicles do not accidentally collide. Another application is a planetary or lunar fly-by of a space probe. The object could be any arbitrary space vehicle or planetary body. If the object vehicle or object is a satellite of a planetary body, both the fly-by vehicle and the object are acted upon by the gravitational force produced by the planetary body about which the fly-by vehicle and object move. When the object is a planetary body, the fly-by vehicle is typically placed on a fly-by trajectory that will cause the fly-by vehicle to fly-by the planetary body so as to use the gravitational force of the planetary body to alter the trajectory of the fly-by space vehicle towards the object. The fly-by approach is a gravity-assisted approach where gravity vectors guide the fly-by vehicle along the fly-by trajectory. The fly-by vehicle is maneuvered into the fly-by trajectory such that gravity vectors will then bring the fly-by vehicle into close proximity to the object vehicle or planet at the desired fly-by distance. For the fly-by mission, a fly-by guidance algorithm has been used at the start of the end phase approach to achieve the desired predetermined fly-by distance. The end phase of a fly-by mission to fly-by the object vehicle is when maneuvering occurs at the start of the end phase to place the fly-by vehicle on a fly-by trajectory under gravitational forces. Planetary fly-by missions have been achieved by remotely estimating the position and velocity state vectors of the fly-by vehicle relative to the planetary object and remotely commanding the fly-by vehicle to maneuver into the fly-by trajectory to achieve the desired fly-by distance. Due to the uncertainties in remotely computed navigational state vectors used to maneuver the fly-by vehicle into the initial fly-by trajectory, the achieved closest approach distance is often far greater than the desired predetermined fly-by distance.
The fly-by gravity assisted approach is different than an intercepting approach such as with a kinetic kill vehicle used to collide with and destroy a target vehicle. The kinetic kill vehicle has an optical sensor for determining a line-of-sight to the target vehicle. The kinetic kill vehicle uses the line-of-sight information in a guidance algorithm to maneuver the kinetic kill vehicle into and along a collision intercept trajectory for colliding with the target vehicle. In the kinetic kill mission, the end phase guidance uses either a continuous or discrete proportional navigation algorithm to maneuver the kinetic kill vehicle onto the intercept trajectory directly towards the target vehicle. The end phase maneuvering is applied to the interceptor vehicle to achieve interception along the line-of-sight trajectory. The line-of-sight sensor is placed on the interceptor to measure the line-of-sight angle as well as the angle rate to the target vehicle used during end phase navigation. Hence, the interceptor requires on-board navigation and guidance using line-of-sight rate data. The line-of-sight sensor and guidance system on the interceptor is used to obtain navigation data to maneuver the interceptor directly towards the target vehicle. Data from the line-of-sight sensor is processed and used to develop proportional navigation maneuvering commands applied to the propulsion system of the interceptor so as to cause a collision with the target vehicle at an effective zero miss distance.
The continuous and discrete proportional navigation algorithms can perform the intercept mission. Proportional navigation maneuvering provides acceleration of the interceptor that is proportional to the line-of-sight angular rate relative between the interceptor and target vehicles. The proportional navigation algorithm is not fuel-efficient because it causes excessive maneuvering during the end phase. Predictive guidance algorithms that predict the future positions of the target vehicle could be used to improve fuel economy by reducing excessive maneuvering during the end phase. The continuous and discrete proportional navigation algorithms use noisy line-of-sight data generated from uncertainties in the sensor and electronics and hence require continuous excessive maneuvering during the end phase. Proportional navigation is designed specifically to guide the interceptor into the target vehicle. Proportional navigation has not been used to achieve a required fly-by distance during a fly-by mission because the proportional navigation algorithm is based upon line-of-sight navigation directly towards the target vehicle. Prior fly-by guidance algorithms do not use line-of-sight sensors for navigation and guidance, and hence, have not used continuous nor discrete proportional navigation. As such, fly-by missions do not have an end phase internal navigation capability that might otherwise provide accurate fly-by distances and fuel-efficient fly-by missions. These and other disadvantages are solved or reduced using the invention.